The invention relates to that aspect of ophthalmic surgery which is concerned with operations upon the external surface of the cornea.
L'Esperance, Jr. U.S. Pat. Nos. 4,729,372 and 4,732,148 describe methods and apparatus for selective and progressive laser ablation of corneal tissue whereby to sculpt a new optically corrected curvature in the optically used portion of the anterior surface of the cornea. Specific illustrations are given for effecting such optically correcting change, in the cases of myopia, hyperopia and astigmatism. The involved principles are applicable for any ablation-producing wavelength, but the specific illustrative disclosure is for use of ultraviolet radiation, in the range of 200 nanometers or less. Reference is made to said patents and to the patent applications referred to therein, for a more complete discussion of tissue ablation and of various techniques for effecting desired corneal-curvature changes.
It suffices for present purposes to explain that the techniques disclosed in said patents may be generically described as employing varying laser-spot size at impingement with the cornea, in the course of a single surgical procedure. According to one mode, a zoom lens is the means of progressive variation of spot size; if the spot is circular and of progressively varying radius, ablation is greater on the optical axis of the eye, and ablation reduces with increasing radius, thus providing a myopia-correcting curvature change in the anterior surface of the cornea. According to another mode, an indexible mask has a progressive development of apertures of varying size which, for the case of myopia correction, are circular and of progressively varying radius.
Each of these modes is subject to at least one limitation which is believed to prevent realization of optimum results. In the case of zoom-lens reliance for spot-size variation, the desired range of spot-size variation can be as much as 5:1 or 10:1, but the greater the magnification range of a zoom-lens system, the greater the range of laser-beam flux density deliverable to the cornea, i.e., flux-density reduction as a function of increasing magnification. In the case of an indexed-aperture mask, the ablated ultimate new curvature is in reality a series of stepped penetrations which only approximate a smooth curvature.
What has been said, as to use of said patents for tissue-ablating laser surgery to reduce myopia, applies also (1) for varying annular-spot projections to reduce hyperopia and (2) for varying slit-width projections to reduce astigmatism.